WDM passive optical networks (PON) have become more and more important as optical distribution networks (ODNs) for distributing optical transmission signals between a central node, e.g. an optical line terminal (OLT) and one or more remote nodes. Often, a bidirectional path in the form of a single fiber connection between the central node and a remote node is used in order to save optical fibers. A bidirectional WDM signal (downstream and upstream WDM signal) is transmitted over the optical link between the central and each remote node. A plurality of optical node units (ONUs) is connected to each remote node. Again, this connection is usually realized by using a bidirectional single fiber connection. In general, only a single channel signal (upstream and downstream channel signal) is used for transmitting the data between the respective ONU and the central node, realizing a point-to-point data link. In order to facilitate the network design, different wavelength bands are used for the downstream and upstream WDM signals between the central node and the remote nodes, and different wavelengths are used for the downstream and upstream channel signals between a remote node and an ONU. Of course, in addition to the bidirectional channel signal, a broadcast signal may be transmitted from the central node to a remote node and to selected or all ONUs.
A plurality of customer signals are aggregated on the trunk or feeder fibers between the central node and a remote node. Therefore, this transmission link needs to be protected. In order to save costs for implementing a protection mechanism, it is desirable to do that without duplicating the optical transceivers in the central node.
An advantageous protection scheme can be implemented by using cyclic 2×N arrayed waveguide gratings (AWGs), i.e. arrayed waveguide gratings having two cyclic WDM ports and a given number of N channel ports. Due to the physics of an AWG, the wavelengths of all channel signals fed to the channel ports must be shifted by one or more channel spacings in order to switch the respective WDM signal from the respective cyclic WDM port to the other cyclic WDM port. The necessary shift depends on the construction of the AWG. In the simplest case, the necessary shift equals a single channel spacing, i.e. the wavelength of each channel signal must be shifted by an amount of a single channel spacing in order to switch the respective WDM signal (including the shifted channel signals) from the respective cyclic WDM port to the other cyclic WDM port. As an AWG has a predefined free spectral range, the channel ports define optical paths for signals at wavelengths differing by an integer multiple of the free spectral range. This property can be used for using the same AWG in order to multiplex or demultiplex downstream and upstream channels that lie in different wavelength bands. For example, a first upstream channel signal may have a wavelength that equals the first downstream channel signal plus or minus the free spectral range of the AWG.
This principle is used in Kwanil Llee et al, “A self-restorable architecture for bidirectional wavelength-division-multiplexed passive optical network with colorless ONUs”, OSA OPTICS EPRESS 4863, Vol. 15, No. 8. This proposed ODN in the form of a WDM-PON includes a cyclic 2×N AWG in the central node and in the remote node. Colorless ONUs are realized by using Fabry-Perot laser diodes in the central node and the ONUs which are wavelength-locked to an injected spectrum-sliced amplified spontaneous emission (ASE) light. For this, two broadband light sources are coupled to a trunk fiber in opposite directions, the trunk fiber being located between the common ports of two optical 1×2 switches, each of the two switched ports of the first 1×2 switch being coupled to a respective cyclic WDM port of the 2×N AWG, and each of the two switched ports of the second 1×2 switch being coupled to a WDM working port and a WDM protection port of the central node, respectively. In the case of detecting, at the central node, a loss of light of the received (upstream) WDM working signal, the wavelengths of the downstream channel signals are shifted so that the downstream WDM working signal is output at the respective other cyclic WDM port of the 2×N AWG. Simultaneously, the two 1×2 switches are controlled to switch to the respective other switched port so that the downstream WDM working signal is transmitted over the protection fiber between the central node, and the remote node and the upstream WDM signal, due to a respective shifting of the wavelength of the Fabry-Perot lasers in the ONUs, is output at the cyclic WDM port of the remote node 2×N AWG being connected to the protection fiber and thus transmitted to the transceivers coupled to the channel ports of the central node 2×N AWG in the desired manner.
However, this WDM-PON network design requires two broadband light sources and two optical 1×2 switches having a corresponding insert loss.
The same is true for WDM-PON architectures using 3 dB couplers in order to split the WDM signals to be transmitted over a working fiber and a protection fiber, respectively (see e.g. Calvin CK Chan et al., “Novel Network Architectures for Survivable WDM Passive Optical Networks”, ECOC 2008, paper Th.1.F.6).
Further, known WDM-PON architectures do not provide for easily increasing the number of channels and the number of ONUs that may be coupled to the remote node (and thus to the central node).